The present practice with injection molding machines is to use the helical screw in the plasticizer barrel as a ram to force material ahead of the screw into the mold. When the screw moves forward to its maximum injection position, it is necessary to maintain it in this position for a period of time which is called the "cure cycle." During the cure cycle, the screw maintains a pressure on the material to compensate for shrinkage of material which is cooling in the mold. Some additional material is forced into the mold to compensate this shrinkage.
The mold area is cooled so that the part being molded becomes solidified. After the cure cycle has been completed, the reciprocating screw plasticizer begins to rotate and forces plastic toward the discharge end of the barrel. The screw continues to rotate until a predetermined amount of material is accumulated at the front of the screw for the next injection operation. The amount of material accumulated is controlled by a limit switch which stops the screw and moves the nonrotating screw back so as to create a suction in the passages leading to the mold. This draws the hot runner material away from the mold and the mold is then opened and the molded part removed.
The objection to this method of operation of the prior art is that the molded part cannot be removed from the mold until the next injection shot has been plasticized and accumulated ahead of the plasticizer screw. The present invention reduces the auxiliary of operation of an injection molding machine by making it possible to remove the molded article much sooner. No drawback of the plasticizer screw is required and the preparation of the next injection charge can be prepared starting immediate after preparatory injection of the previous charge and without waiting for any part of the cure cycle to elapse.
This invention provides a valve between the mold and the discharge end of the plasticizer valve. An auxiliary cylinder and piston is used to maintain pressure on the material in the mold during the cure cycle and the piston in this auxiliary cylinder is reversed in direction to draw back the runner material prior to discharge of the molded part from the mold. Thus it is possible, with this invention, to open the mold, remove the molded part, and close the mold preparator to the next operation while the plasticizer screw is plasticizing the next quantity of material for the molding of another part. This greatly reduces the overall cycle of the apparatus.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.